DEFINITIONS OF DUNE SAND AND LAG GRAVEL 351 



Dune sand is the wind-blown sand of much coarser texture than the 

 loess. Its coarser particles are pushed along the ground, while the finer 

 form clouds of dust in the air which settle rapidly or slowly, near to or 

 remote from the source of supply, according to the force of the wind and 

 the size of the particles suspended. 



Lag gravel is the coarser material of worn or subangular habit which 

 moves heavily along the ground, pushed by violent winds through short 

 distances. This material sustains the same relation to the dune sands 

 as the latter do to loess. Hence we have in these three characteristic 

 accumulations three different phases of eolian deposits, comparable in 

 their several relations to each other to three characteristic accumula- 

 tions under water, namely, gravel sand, and clay. 



The identification of the eolian deposits therefore depends on skill 

 in distinguishing between wind deposits and water deposits. 



Loess 

 general characteristics of minnesota loess 



The loess as a deposit in this region is never more than a thin veneer 

 seen occasionally on the higher hills. It is therefore scarcely a typical 

 loess, since being near the surface the humic acids have from the time it 

 was laid down had ready access to it. The deposit in most places has 

 been entirely reduced or exists in the soil only. When two or three feet 

 thick, as it sometimes is, it is distinguishable from the till only on close 

 ins[)ection, the slight differences in color and composition needing the 

 additional presence of pebbles in the till and a stratified arrangement 

 when "modified " to make discrimination easy. Whenever it occurs in 

 thinner layers there is but little material that can be distinguished from 

 the associated soil. The wind-borne dust was not deposited rapidly 

 enough to produce deep and widespread beds of loess. 



LOESS IN SAINT PA UL 



■ As a typical locality a bed of loess in Saint Paul is shown on plate 33, 

 figure 1, and briefly described. It is one of many such thin layers to be 

 seen throughout the eastern portion of the state. On Como avenue, near 

 the Como-Harriet interurban electric railroad, is an exposure showing 

 the following sequence downward : v 



1. Dark colored soil from 4 to 8 inches in thickness, supporting the usual amount 

 of vegetation. 



2. Loess of alight pinkish gray color, wholly without stratification, fine and non- 

 indurated in texture. Its thickness varies from 2 to 3 feet. It is distinguished from 

 the till by the entire absence of pebbles or layers of sand. 



3. Clayey till. For 5 feet of its thickness this layer is a fair quality of clay, but 

 it gives place downward to a bed of gravelly till. 



